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Mina Gheiratmand, Kathy T. Mullen; Orientation tuning in color vision measured in the absence of contrast gain control. Journal of Vision 2012;12(9):600. doi: https://doi.org/10.1167/12.9.600.
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It is thought that measuring response tuning using sinewave masking confounds the effects of the within channel response with the more broadly tuned cross-channel gain controls activated by the use of high contrast mask stimuli (Cass et al., JOV 9(12), 2009; Meese & Holmes, JOV 10(12), 2010). Here we use a method of subthreshold summation to measure the orientation tuning of color vision at very low contrasts, without the influence of contrast gain controls. Methods: Stimuli were oriented sine‐wave gratings (sf = 1.5 cpd) in a circular patch (10 degs) with rasied cosine edges. Gratings were presented alone or as overlaid pairs to form a plaid. The orientation difference between the two gratings was varied from 0 to 90 degs. For each plaid angle, psychometric functions were obtained for the detection of each grating and the "plaid", and a summation ratio was determined. Orientation tuning was measured for isoluminant red-green and achromatic stimuli presented monocularly. Results: Summation ratios as a function of orientation difference were fitted with Gaussian functions with an offset parameter (~1.3 dB) to take into account probability summation. At this spatial frequency, we find orientation-tuned responses for color vision that are similar to those obtained for achromatic vision with a bandwidth (HWHH) of 12.0 ± 3.8 degs for color and 9.0 ± 2.4 degs for achromatic stimuli (average of three subjects). Conclusion: Subthreshold summation reveals orientation tuning in color vision at mid spatial frequencies, whereas masking reveals very broad or isotropic effects (Medina & Mullen, JOV 9(3), 2009). This difference is likely due to the influence of contrast gain control in the masking paradigm. Orientation tuning for color contrast that matches that for achromatic contrast supports a primary role for color vision in shape perception.
Meeting abstract presented at VSS 2012
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